In a time-of-flight mass spectrometer, the mass of an ion is generally calculated from the time of flight which is obtained by measuring a period of time required for the ion to fly over a fixed distance, on the basis of the fact that an ion accelerated by a fixed energy has a flight speed corresponding to the mass of the ion. Accordingly, elongating the flight distance is particularly effective to enhance the mass resolution. However, elongation of a flight distance simply on a straight line requires unavoidable enlargement of the device, which is not practical, so that a variety of ion optical systems for forming an ion flight space have been developed.
One known type of such an ion optical system is a multi-turn optical system in which a plurality of sector-shaped electric fields are used to form a closed orbit such as a substantially elliptical orbit, substantially “8” figured orbit, etc (refer to Patent Document 1 and other documents, for example). Ions are made to fly along such a loop orbit repeatedly multiple times to elongate the flight distance.
In this type of multi-turn time-of-flight mass spectrometer, it is necessary to prevent a decrease in the sensitivity and resolution due to temporal and spatial expansion of ions having the same mass-to-charge ratio during their flight along the orbit. Therefore, in designing the ion optical system to form a loop orbit, it is required that the time-of-flight peak should not be broadened and the ion beam should not be diverged after the flight, in addition to the requirement that the orbit should be geometrically and structurally closed. In the explanations below, an ion optical system for foaming a loop orbit will be simply called an ion optical system.
To meet such demands, in the multi-turn time-of-flight mass spectrometer described in Patent Document 1, for example, it is required as a time-focusing condition that the time of flight of the ions after the flight through the loop orbit is not dependent on an initial position, initial angle, and initial energy of the ions at the moment when they start to fly. Such conditions limit the shape and arrangement of sector-shaped electric fields to configure the ion optical system, and therefore the design of the ion optical system is not always easy.
Increasing the number of turns on the loop orbit enhances the mass resolution. However, in the case where ions having different masses are mixed, ions having a smaller mass and flying faster catch and pass ions having a larger mass and flying more slowly, which makes it difficult to distinguish the ions. Given this factor, in order to enhance the mass resolution, it is desirable to elongate the distance of one cycle of the loop orbit as much as possible so that ions do not catch and pass ions having different masses. The elongation of the distance of one cycle requires an increase in the number of sector-shaped electric fields which compose the ion optical system, an increase of their curvature, and an elongation of the length of free flight spaces. In the end, this requires an enlargement of the installation area of the ion optical system.
One method for preventing ions from catching and passing other ions on the loop orbit, and moreover, for suppressing the installation area is to form a helical flight orbit. In the apparatuses described in Non-Patent Documents 1 through 3, for example, a loop orbit which is stable on a plane and capable of focusing a variety of spreads (or dispersions) that ions have is slightly deflected in the direction perpendicular to the plane to form a helical orbit. With such a configuration, even if the focusing (particularly time-focusing) condition of ions is satisfied with regard to the loop orbit on plane, the focusing condition of ions with regard to the entire helical orbit is not assured. In particular, an increase in the number of turns to elongate the flight distance might pose a problem in that some ions disperse to decrease the sensitivity or the mass accuracy and mass resolution are not increased as much as expected.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. H11-297267
[Non-Patent Document 1] H. Matsuda, “Improvement of a TOF Mass Spectrometer with Helical Ion Trajectory,” Journal of Spectrometry Society of Japan, 49, pp. 227-228 (2001)
[Non-Patent Document 2] H. Matsuda, “Spiral Orbit Time of Flight Mass Spectrometer,” Journal of Spectrometry Society of Japan, 48, pp. 303-305 (2000)
[Non-Patent Document 3] T. Satoh and three other authors, “A New Spiral Time-of-Flight Mass Spectrometer for High Mass Analysis,” Journal of Spectrometry Society of Japan, 54, pp. 11-17 (2006)